Numerical field evaluation of healthcare workers when bending towards high-field MRI magnets

In MRI, healthcare workers may be exposed to strong static and dynamic magnetic fields outside of the imager. Body motion through the strong, non-uniform static magnetic field generated by the main superconducting magnet and exposure to gradient-pulsed magnetic fields can result in the induction of electric fields and current densities in the tissue. The interaction of these fields and occupational workers has attracted an increasing awareness. To protect occupational workers from overexposure, the member states of the European Union are required to incorporate the Physical Agents Directive (PAD) 2004/40/EC into their legislation. This study presents numerical evaluations of electric fields and current densities in anatomically equivalent male and female human models (healthcare workers) as they lean towards the bores of three superconducting magnet models (1.5, 4, and 7 T) and x-, y-, and z- gradient coils. The combined effect of the 1.5 T superconducting magnet and the three gradient coils on the body models is compared with the contributions of the magnet and gradient coils in separation. The simulation results indicate that it is possible to induce field quantities of physiological significance, especially when the MRI operator is bending close towards the main magnet and all three gradient coils are switched simultaneously. Magn Reson Med 59:410-422, 2008. © 2008 Wiley-Liss, Inc

Reverse-engineering of gradient coil designs based on experimentally measured magnetic fields and approximate knowledge of coil geometry-application in exposure evaluations

For many MRI installations, the coil pattern that generates pulsed magnetic fields produced by gradient coils is not provided by the manufacturer. This has implications for accurate assessments of MRI worker exposures, which is currently an important topic of research. To correctly model the level of exposure, a full three-dimensional distribution of the magnetic field in the locality of the magnet end is required, which can be difficult to obtain by experimental measurements. This research presents one possible approach, in which the prediction of a current distribution that generates an approximately identical magnetic field profile is constrained by a small number of experimentally measured magnetic field sample points within a plane outside the gradient set. The presented methodology may take into consideration other important descriptors such as field uniformity in the imaging volume, gradient coil geometry/dimension, driving current, central field strength, etc. To exemplify the application of the proposed approach, the current density and matching magnetic field distributions of x- and z-axis gradient coils are approximated without preknowledge of the gradient coil patterns for a MRI system, followed by exposure evaluations of a tissue-equivalent numerical worker model in the vicinity of said gradients. (C) 2009 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 3513: 32-43, 200